Saturday, September 22, 2007



Tomorrow morning at 4:51 am CDT (9:51 UT) the fall equinox will arrive.You can convert this to your own time zone using the Time and Date site. The equinox is defined as the time when the Sun is directly above the Earth's equator. This usually occurs around March 20th (the Vernal Equinox) and September 22nd (the Autumnal Equinox). At this time the Sun crosses celestial equator. The equinox is also the time of year when the celestial equator intersects with the ecliptic. The equinox is a point in time, not the name of a day. The day when daylight and nighttime are "most nearly equal" is referred to as the Equilux. while the equinox falls on September 23rd this year the equilux will actually be September 26th.

The time of the equinox isn't fixed. Each year they fall about 6 hours later. This is partially corrected by the leap year every four years which resets the time of the event. The reset isn't perfect however, and there is a slow drift of the time of the equinoxes (and solstices) to earlier times in the year. The shift amounts to a full day over the course of about 70 years. This shift is largely compensated by the century leap year rule of the Gregorian calender.

As we approach the equinox the rate of change in the length of a day increases. If you follow the time periods of daylight you will notice that they change most slowly around the times of the solstices but change most rapidly at the equinoxes. At the poles this means that the rate of change is instantaneous. The equinox marks the transition from 24 hrs sunlight to 24 hrs darkness. The further you travel from the poles the slower the rate of change. At the equator the rate of change is mere seconds per day. This apparent "midnight sun" in its full effect can be seen up to 100 kms from the poles. The time from the solstice in June to the September equinox is 94 days. The time from the December solstice to the March/Vernal equinox is only 89 days. This discrepancy arises because the orbit of the Earth is elliptic rather than perfectly circular. This means that the rate at which the Earth orbits the Sun also varies. It is faster towards the spring(northern hemisphere) when the Earth is closer to the Sun.

The actual times of daylight and night are not equal at the time of the equinox. This is not just because the equinox is a point in time. It is also due to other factors that contribute to the day being longer than the night at this time of year. First of all, the Sun is not a point source of light. It is a disc, and sunrise and sunset are defined from the point of view of the upper edge of the disc. The discrepancy is at least a minute on both ends of the day. There is also the effect of light refraction when the Sun is near the horizon. This makes the Sun seem to be a little bit more above the horizon at both sunrise and sunset. This effect adds almost seven minutes to the daylight. If you take twilight into account the "day" at the time of the Equinox would be almost an hour longer than the night. As you go towards the poles this difference increases, and the time of the equinox has much more day than night.


There are a number of other interesting things about the equinoxes:

*Equinoxes have a temporary disruptive effect on geostationary communications satellites. This happens because there is a point in time at the equinoxes when the Sun is directly behind the satellite from the point of view of receiving stations on Earth. The Sun's radiation overwhelms the much weaker signal from the satellite with noise. The duration of this effect varies. it may last only a few minutes, but it may persist up to an hour.

*There are a number of other names for the two equinoxes rather than spring/Vernal equinox and autumnal/fall equinox. These names suffer from the fact that they are obvious references to matters only in the northern hemisphere. South of the equator the situation of the seasons is precisely the opposite. Some have proposed using 'March Equinox' and 'September Equinox'. This is familiar to those who use the western solar based calender, but lunar calenders such as the Jewish or Muslim calenders have the equinoxes falling in different months from year to year. The equinoxes were once named in astrology as the 'First Point of Aries' and the 'First Point of Libra'. Because of precession these astrological signs are no longer the constellations where the equinoxes actually occur. Today they are the 'Pisces Equinox' and the 'Virgo Equinox'. One hardly ever hears such terms in common usage. Finally, there are the 'Northward Equinox' and the 'Southward Equinox', referring to the direction of the apparent motion of the Sun at each equinox. These terms are also rarely used.

*There is a persistent folk legend that the equinoxes are the only time of the year when you can stand an egg on end. This myth has been thoroughly debunked by the owner of the Bad Astronomy Blog who devotes a lot of his efforts to exposing hoaxes, myths and misconceptions in the area of astronomy. In actual fact you can stand an egg on end at any time of year. It just takes a little practice and skill. See the article at

*Because the Sun is not a point source of light it actually takes the Sun about two and 1/2 days to cross the equator. The equinox is defined as the time when the midpoint of the Sun's disc is over the equator.

*A couple of good references on things equinoxal:

1)Details about the length of day and night at the equinoxes.

2)Calculation of length of day.

3)Table of times for equinoxes, solstices, perihelion and aphelion in 2000-2020.


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